1. Technical Field
The present invention relates to an ink for inkjet recording (hereinafter referred to as an “ink”), an ink cartridge, and an inkjet recorder.
2. Description of the Related Art
Inkjet recording methods have been popular as image forming methods because of having advantages of having simpler process and easier full-colorization than the other recording methods, and producing high-resolution images even with an apparatus having simple composition. The inkjet recording methods have a small amount of ink soar and adhere to recording media such as papers to form images thereon with an inkjet recorder, and applications thereof are expanding, e.g., personal and industrial printers and printings.
In the inkjet recording methods, an aqueous ink using a water-soluble dye is mostly used as a colorant. However, the ink has disadvantages of having poor weatherability and water resistance. Therefore, a pigment ink using a pigment instead of the water-soluble dye has been studied recently. However, the pigment ink is still inferior to the dye ink in colorability, ink discharge stability and preservation stability.
In company with improvement of higher-quality image technology of OA printers, even when recorded on plain papers as recording media with the pigment ink, image density equivalent to that of the dye ink is required. However, the pigment ink penetrates into a plain paper as a recording medium and pigment density at the surface of the paper lowers, resulting in lower image density.
In order to dry the ink adhering to the recording medium quicker to print quicker, a penetrant is added to the ink for water to penetrate into the recording medium. Then, not only water but also pigment penetrates deeper into the recording medium, resulting in lower image density.
When a plain paper is used as a recording medium, the surface of the plain paper swells due to water which is a solvent of the ink and a difference of extension rate between the front side and the back side of the paper becomes large, resulting in curl. This has not been a problem in low-speed printing because of being solved as the paper is dried.
However, in high-speed printing, paper jam may occur because a recording medium is fed with curl unsolved. An organic solvent in the ink is effectively increased to solve curl. However, it becomes difficult to assure storage stability of the ink because of being more hydrophobic.
In order to solve this problem, PCT Japanese published national phase application No. 2009-513802 discloses an inkjet composition including a liquid vehicle, a colorant, and a polymer having at least one functional group having a specific calcium index value. Monomers forming the polymer include 4-methacrylamide-1-hydroxybutane-1,1-diphosphonic acid. The colorant is destabilized by the diphosphonic acid group and Ca salt in a paper when contacting the paper, and the resultant printed image improves in quality.
Japanese published unexamined application No. JP-2012-51357-A discloses an inkjet recording method of applying a receiving liquid including a Ca salt; and an ink including a pigment combined with a group having phosphorus, a resin emulsion and a surfactant thereon. It is disclosed that a bisphosphonic acid is preferably used as the group having phosphorus, which is reacted with the Ca salt in the receiving liquid to improve feathering and fixability. However, a polymer including phosphonic acid group is not used as dispersant or an additive in a pigment dispersion, and a pigment combined with a phosphoric acid is used. Therefore, image density of an image recorded on a plain paper is not sufficiently improved. Further, the surface of a pigment needs chemical decoration, and the selection thereof is limited.
Japanese published unexamined application No. JP-2004-123904-A discloses an aqueous ink including a colorant, water, hydrosoluble organic solvent, a surfactant and a chelate agent. A low-molecular-weight hydroxy ethylidenediphosphonic acid or its salt is used as the chelate agent. The chelate agent removes calcium included in a pigment dispersion to improve discharge stability and storage stability of the ink. However, only the low-molecular-weight hydroxyethylidenediphosphonic acid is disclosed, and a polymer including a phosphonic acid is not disclosed. A relation between the chelate agent and improvement of the image density of an image when recorded on a plain paper is not disclosed, either.